High molecular weight polyethylene fibers and processes for their manufacture are known in the art.
U.S. Pat. No. 4,344,908 describes a process for making polymer filaments which have a high tensile strength and a high modulus by stretching a solvent-containing polymer filament at a temperature between the swelling point and the melting point of the polymer.
EP 231 547 describes a process for manufacturing high strength high modulus polyethylene objects by polymerising ethylene in a hydrocarbon solvent in the presence of a catalyst system to form a solution of linear polyethylene with a molecular weight of 4*105 to 5*106 gram/mole, converting the solution to a solvent-containing object such as a fiber, cooling the object to form a gel, and subjecting the object to a stretching step.
US 2004/0267313 describes a method in which a gel-spun ultra-high molecular weight polyethylene is subjected to a processing step to remove solvent before or after the fiber is woven with other fibers.
A disadvantage of the methods described above is that they all involve the use of solvent in the manufacture of the polymer. Accordingly, the thus obtained fibers always contain a certain amount of residual solvent, which may detrimentally affect the properties of the fiber. Further, the recovery of solvent is highly uneconomical.
Solvent-free methods for manufacturing fibers from high molecular weight polyethylene have also been described.
JP6010254 describes a method of manufacturing a high strength non-woven fabric composed of an ultrahigh molecular weight polyethylene. A film of the ultra-high molecular weight polyethylene is subjected to a slitting step, and the slit film is stretched with a stretch ratio of at least 2 in at least the longitudinal direction to provide stretched tape. The stretched tape is cooled to a temperature below 80° C., and then opened at an opening ratio of 0.5-4 (roll peripheral speed/tape speed), to provide split yarn. The split yarn is then formed into webs, which are joined to form the non-woven material. The slitting process applied in this reference has a number of disadvantages. To name a few, there is a minimum width for the strips obtained by the splitting, the splitting step may detrimentally affect polymer properties, and the slitting step is an extra processing step and desirably avoided for that reason also. It is known in the art that low-linear density fibers add more comfort, flexibility, and elongation to products like ropes and fabrics. The relatively wide slitted tape as mentioned does not provide such an advantage.
U.S. Pat. No. 5,578,373 describes a method for manufacturing a split polyethylene stretched material by subjecting an ultra-high molecular weight polyethylene to stretching and then subjecting the stretched polyethylene to splitting. While methods like tapping, twisting, rubbing, brushing, the use of an air yet, and the use of ultrasonic waves and bomb blasts are mentioned, mechanical splitting methods which use splitters of various types are preferred.
US2003/0127768 describes a process for the manufacture of a shaped part of ultra-high molecular weight polyethylene via melt-processing wherein ultra-high molecular weight polyethylene is annealed at a temperature between 130 and 136° C. for at least one hour, converted into a shaped part at a temperature above 142° C., and then cooled down to a temperature below 135° C. To form a fiber, the annealed material can be spun through a spinneret to form filaments, which are subsequently stretched at a temperature between the melting point of the filament and a temperature not more than 10° C. below the melting point. Again, this process has a number of disadvantages. The process of this reference comprises an annealing step which is desirably avoided. Further, the spinning of ultra-high molecular weight polymer melts requires detailed process control, int. al. as a result of the high viscosity of molten ultra-high molecular weight polyethylene, and is therefore not easy to operate in commercial practice.